This invention relates to a process for treating (or cleaning) surfaces of semiconductors, LCD, etc. with special treating solutions containing a special complexing agent and the treating solutions per se.
Semiconductor devices such as LSI, IC, diodes, commutators, etc. are almost silicon devices which are produced by subjecting a silicon wafer to processing steps such as vapor phase growth, oxide film formation, impurity diffusion, vapor phase deposition of electrode metal, etc. Since contamination with impurities remarkably influences on electrical properties of semiconductors, it is necessary to clean silicon wafer surfaces sufficiently for removing contamination prior to above-mentioned processing steps.
Various industrial cleaning methods which are different as for treating chemicals, treating temperature, etc. have been employed. A series of treatments called "RCA cleaning method" (RCA Review pp. 187-206, June, 1970, etc.) appeared in the mid-1970s and have been used widely since that time and become a major method for wet cleaning. The RCA cleaning method is a combination of (i) SC-1 treatment using ammonia, hydrogen peroxide and water and effective for removing contamination with organic materials and a part of metals such as Cu, Ag, etc., particularly effective for removing contamination with fine particulates, (ii) a dilute HF treatment for removing natural silicon oxide films, and (iii) SC-2 treatment using hydrochloric acid, hydrogen peroxide and water, and remarkably effective for removing metal contamination.
The sequence of RCA cleaning method generally used is SC-1 treatment.fwdarw.dilute HF treatment.fwdarw.SC-2 treatment. But the dilute HF treatment easily gives fine particulates contamination on the surface of silicon wafer and the SC-2 treatment is poor for removing fine particulates. With recent ultra-high integration of LSI, device patterns are rapidly miniaturized and the size of fine particulates which damage devices is also remarkably miniaturized. The smaller the size of fine particulates becomes, the stronger the adhesive strength for wafers becomes. Thus, the removal of fine particulates becomes more difficult, resulting in making the production yield strongly depending on the removal ability of fine particulates contamination at the time of cleaning. In order to remove the particulates most effectively, it was tried to conduct the SC-1 treatment at the last step of cleaning sequence. But in case a wafer has been subjected to such a cleaning sequence, the surface of wafer was contaminated with metallic impurities and there arised problems in electrical properties such as deterioration in oxide breakdown voltage and recombination lifetime, etc.
Metallic elements detected on the surface of wafer after the SC-1 treatment are Fe, Al, Ca, Mg, Zn, etc. At first these contaminating metals are derived from the production apparatus for the processing step before the cleaning step or derived from circumstances. They consist of the elements which are not removed due to insufficient cleaning ability of SC-1, and the elements adsorbed on the wafer from the treating solution including these metal elements.
Metallic impurities difficult to be removed are easily adsorbed on the surface of wafers in general. Among various metallic impurities, Fe and Al have such a tendency remarkably as compared with other above-mentioned elements. In a mass production factory, as a high-productivity cleaning apparatus, there is commonly used a Tact method wherein wafers entered in a carrier are passed to cleaning baths one after another. Thus, contaminating elements released in the liquid of a SC-1 treating bath by cleaning are gradually accumulated to provide an undesirable contamination by adsorption on succeeding wafers. Further, when ammonia and hydrogen peroxide with extremely high purity are not used, there takes place contamination by adsorption of impurities in the cleaning chemicals. In the production and storing of hydrogen peroxide, since metals such as Al are used, it is particularly necessary to highly purify the hydrogen peroxide used for semiconductor cleaning. Further, these highly pure chemicals are easily contaminated in vessels for transport or in feeding system to cleaning baths, that it is not easy to maintain the purity of these chemicals at very high level in cleaning baths.
Usually, after the SC-1 treatment, the surface of wafer is contaminated with about 10.sup.11 to 10.sup.12 atoms/cm.sup.2 of Fe, about 10.sup.11 to 10.sup.13 atoms/cm.sup.2 of Al, and about 10.sup.10 to 10.sup.11 atoms/cm.sup.2 in the cases of Ca, Mg, and Zn. Contamination with these metals in such concentrations except for Fe gives negligible influence of electrical properties such as oxide breakdown characteristics and recombination lifetime properties. Thus, the element which damages electrical properties as for the SC-1 treatment is Fe. On the surface of the p-type wafer in which influences of Fe on the lifetime is remarkable, negligible concentration of Fe for lowering the lifetime is 10.sup.10 atoms/cm.sup.2 or less. Further, as to a time dependent dielectric breakdown test (TDDB properties) of oxide films, it is necessary to make the concentration of Fe as low as about 10.sup.10 atoms/cm.sup.2 in order to attain an intrinsic breakdown time of non-contaminated wafer. It was extremely difficult to obtain such a degree of cleanness only by the SC-1 treatment.
Another cleaning method similar to the SC-1 treatment is a method of using an organic alkali and hydrogen peroxide. For example, there are disclosed tetramethylammonium hydroxide (TMAH) and hydrogen peroxide (Japanese Patent Unexamined Publication No. 50-147284), trialkyl(hydroxyalkyl)ammonium hydroxide and hydrogen peroxide (Japanese Patent Examined Publication No. 53-43012), etc. These methods are excellent in fine particulates removing ability which is characteristic of the treatment with alkali and hydrogen peroxide like the SC-1 treatment, but great in adsorption of Fe, Al, etc. from the treating solution, resulting in making cleaning ability for wafers contaminated with Fe, Al, etc. insufficient. A method of inactivating metallic impurities as to a material to be cleaned by catching them as stable water-soluble complex is a conventional method for formulating usual cleaning agents. For example, addition of a complexing agent to TMAH and hydrogen peroxide is disclosed in Japanese Patent Unexamined Publication No. 50-158281, and addition of a complexing agent to trialkyl(hydroxyalkyl)ammonium hydroxide and hydrogen peroxide is disclosed in Japanese Patent Examined Publication No. 53-20377 (U.S. Pat. Nos. 4,239,661 and 4,339,340). In each case, the added amount of the complexing agents is in need of 0.01% by weight or more. Addition of a cyano compound such as ammonium cyanide, hydrogen cyanide is dangerous. When an organic compound such as ethylenediamine tetraacetic acid (EDTA), triethanolamine, etc. is added in an amount of 0.01% by weight or more as described in them, harmful carbon contamination takes place on silicon surface to cause problems in electrical properties. Further, when silicon surface is contaminated with Al, it is known that oxidation rate during thermal oxidation is influenced. This is not preferable from the viewpoint of precise control of the process.
As mentioned above, no effective methods have been found to solve insufficiency in cleaning efficiency for harmful metallic impurities such as Fe, Al, etc. in the "alkali and hydrogen peroxide" cleaning which is very effective for fine particulates contamination.